1. Field of the Invention
The present invention relates to semiconductor wafer processing systems, and more particularly, to systems for etching semiconductor wafers in a thermally controlled environment.
2. Description of the Background Art
A semiconductor wafer processing system that performs dry etching of semiconductor wafers typically accomplishes the etching within a process chamber. The chamber is a vacuum sealed enclosure containing a wafer pedestal for mounting a wafer in a stationary position within the chamber during the etch process. To plasma enhance the etch process, a plasma is generated within the chamber by filling the chamber with a reactant gas, and applying a substantial RF field to the reactant gas to generate a plasma. The RF field is generated by conductive coils that circumscribe the outer circumference of the chamber as well as a cathode positioned within the chamber. These coils form an antenna that is driven by a high powered RF signal to produce a substantial magnetic field within the chamber. The cathode is also driven by an RF signal that produces a substantial electric field within the chamber. The magnetic and electric fields interact with the reactant gas to form a plasma within the chamber. The antenna is positioned at a location on the exterior of the chamber to ensure that the plasma is uniformly generated above the wafer surface that is to be etched.
To reduce the temperature change that is experienced by the chamber when the plasma is cycled on and off, the dome is typically heated to approximately 80xc2x0 C. using a radiant heat lamp source. The radiant source is generally a plurality of high-power lamps mounted outside of the chamber. The lamps are mounted in an array above the dome. Typically, a reflector assembly is positioned proximate the lamps to focus their radiant energy upon the dome surface. To maintain the dome temperature at approximately 80xc2x0 C. during plasma cycling (i.e., during periods when the plasma is present and not present), a fan is mounted proximate the dome to provide a continuous flow of room temperature air across the dome and thereby maintaining the dome at a constant temperature when the plasma is present. However, such a cooling technique is not very effective and the dome temperature may fluctuate as much as xc2x140xc2x0 C. depending upon the ambient room temperature. Large thermal fluctuation of the chamber surfaces cause the chamber to expand and contract such that material deposited on the chamber walls and dome during the etch process flakes and falls upon the wafer being processed. The microcontamination particles makes the wafer unusable.
Therefore, a need exists in the art for a dome temperature control apparatus that maintains the temperature of the dome within at least xc2x15xc2x0 C. of a preset nominal temperature during cycling of the plasma.
The disadvantages heretofore associated with the prior art are overcome by the present invention of a closed-loop dome temperature control apparatus. The apparatus contains a centrally-located, high-volume fan, a heat exchange chamber and an enclosure that encloses both the fan and the heat exchange chamber to form a closed-loop air flow circulation system. The closed loop system circulates air within the enclosure to stabilize the temperature of a dome of a semiconductor wafer processing system. To facilitate optimal air flow over the dome, the apparatus contains an air flow director.
In a first embodiment of the invention, the fan blows air from above a lamp assembly downward through a central portal defined by the lamp array assembly. The fan also blows air across the circumference of the lamp array assembly and through a vortex generator (one embodiment of an air flow director) containing a circular array of air directing nozzles. These nozzles direct the air flow toward the dome in a circular fashion generating a vortex or cyclone of air that swirls about the top of the dome. As the air swirls past the dome, it exits the edge of the dome into a heat exchange chamber wherein a plurality of tubes carrying a heat transfer fluid to chill or heat the air. The air passes through the heat exchange chamber over a fan shroud and back to the fan which again pushes the air down across the lamps and the upper portion of the dome. As such, a closed-loop, dome temperature control apparatus is produced.
In a second embodiment of the invention, an axial flow fan module blows air from above a lamp assembly through a central portal defined by the lamp assembly. The air flows through an air flow director containing a circular array of stationary air directing blades (i.e., stator blades).
These blades direct the air flow along an axial path toward the dome to provide a uniform cascade of air that, upon impact with the dome, flows radially outward over the top of the dome. As the air flows past the dome, it exits the edge of the dome into a heat exchange chamber wherein a plurality of finned tubes carry a heat transfer fluid to chill (or heat) the air. The air passes through the heat exchange chamber as the air flows back to the fan which again pushes the air down toward the center portion of the dome. A series of upper stator blades located above the fan are also utilized to prevent back flow of air from the axial fan.
Using this invention of a closed-loop apparatus, the dome temperature can be maintained to xc2x15xc2x0 C., about a nominal temperature of 80xc2x0 C. The temperature can further be controlled to raise or lower the nominal temperature by adjusting the power that is delivered to the lamps. The lamp control is facilitated by an infrared sensor that is focused upon the surface of the dome. The sensor signal is processed through a conventional software implementation of a feed-back loop. The feedback loop utilizes a preset temperature value to which the measured temperature is compared. The closed loop system controls the current delivered to the infrared lamp array to cause the measured temperature to equal the preset temperature value.